The present invention relates to a magnetic resonance imaging (MI) system comprising a housing defining an examination space for receiving a body for examination, a magnetic field generating system for generating a magnetic field in the examination space, and an electromagnetic resonance receive system which comprises at least one dedicated receive coil.
MRI technology is generally known, and does not need to be explained in great detail here. Suffice it to say that this technique involves applying an electromagnetic field to a body under examination such that the magnetization vector in the imaging volume is rotated. After the RF excitation has been removed, the magnetization vector is precessing about the B0 field lines at the Larmor frequency, thus causing RF magnetic resonance signals that can be received by RF receiving coils.
MRI systems generally comprise an examination space for receiving a body for examination, such as a patient, and a magnetic field generating system for applying the required magnetic field to the examination space. Such magnetic field generating system typically comprises a static magnetic field coil for generating a static field or B0 field, one or more gradient coils for generating a gradient field, and one or more excitation coils for generating an RF magnetic field or B1 field. This magnetic field generating system is arranged in a housing, arranged close to the examination space. In a typical example, the B0 field has a direction parallel to the longitudinal axis of a body to be examined, and the housing of the magnetic field generating system has a substantially annular shape extending around the examination space. In such case, the examination space is also indicated as “bore”. In another typical example, the B0 field has a direction perpendicular to the longitudinal axis of a body to be examined, and the housing of the magnetic field generating system comprises an upper housing and a lower housing each having a substantially disc-shaped contour, arranged above and below the examination space, respectively. In such case, the system is also indicated as “open system”.
MRI systems further comprise receive antennas for receiving the electromagnetic radiation transmitted by the relaxing particles. One or more of the transmit coils of the magnetic field generating system may be used as a receive antenna. A typical example is a quadrature whole-body transmit-receive coil, also indicated as QBC.
For improved imaging performance, special antenna elements have been developed, which are intended to be placed close to the body under examination. Which antenna element to be used depends on the body part to be examined. These special antenna elements are also indicated as local coil or dedicated receive coil DRC. It is noted that in some systems only local coils are used as receive antenna, i.e. the receive system does not comprise any of the transmit coils of the magnetic field generating system.
In the state of the art, these dedicated receive coils are separate elements, which are placed on the body by medical personnel. As such, the use of such dedicated receive coils is associated with some problems. A first problem relates to the imaging process. Imaging involves processing the signals picked up by the antenna elements and generating images which can be suitably viewed by people, especially medical staff. Positioning of the antenna elements by hand may be a source of errors in that the actual position of an antenna element does not correspond sufficiently with the expected position, because the antennas are placed on the outside of the patient and the object to be imaged is on the inside.
Said signal processing is performed by a specialized computer. Some of the image reconstruction methods in the software in this computer have been developed under the assumption that the antenna elements have a defined position.
This problem is especially important in case it is intended to perform imaging according to the SENSE method. This method is known per se to persons skilled in the art. By way of example, reference is made to a descriptive article “SENSE: Sensitivity Encoding for Fast MRI” by Klaas P. Pruessmann in Magnetic Resonance in Medicine, 42, 1999, p. 952-962. The sense method requires that the position of the antenna elements be accurately known. To this end, the state of the art requires that, before the actual examination, a reference scan be made, producing information on the actual position of the antenna elements. A complicating factor is that, in order for this information to be valid in the actual patient examination, the patient is not allowed to move (in an undefined manner) between the reference scan and the examination scan.
A further problem relates to the mere fact that arranging dedicated receive coils requires the presence and time of trained medical personnel. Also, arranging the dedicated receive coils must be done at the MRI system, i.e. this procedure occupies the system which at the time is idle, so that the system cannot be used in a cost-efficient way.
A further problem relates to the fact that the process of arranging dedicated receive coils, and removing them later, involves handling which may cause damage to the coils, resulting in a possible need for repair (costs) and the risk of being out-of-service for some time.
A further problem relates to the fact that the antenna elements may be a hindrance when it is desired to arrange other sensors on the patient's body, for instance for generating ECG signals.
A further problem relates to patient-friendliness. The requirement of lying motionless between two scans has already been mentioned. Also, during a scan, a patient should not move since this might shift the dedicated receive coils. Apart from that, the fact that a number of antenna elements is arranged on the patient's body in uncomfortable for the patient, and the patient may even feel confined.
Nowadays, there is an increasing demand for whole-body scans, i.e. imaging of the whole body of the patient. In the state of the art, whole-body imaging with dedicated receive coils requires that the whole body of the patient be covered with a large number of coils. Not only is this very uncomfortable to the patient, but it is also labour-intensive for the personnel who must place and remove the coils in respect of each patient or scan. Further, it involves handling many thick cables and many connectors. With a view to safety, it is desirable to reduce the number of cables close to the patient.
U.S. Patent application 2002/0138001 A1 discloses an MRI system having at least one local coil secured to a movable carrier, which carrier is connected to a carrier mount which is arranged stationary in the examination space. In use, the patient is positioned in the examination space, and then the carrier is moved, controlled from a remote position, to push the local coil against the patient's body. Although such systems will reduce some of the problems mentioned above, it still leaves some problems or even introduces others.
For instance, the need for a movable carrier increases the complexity and costs of the system.
Further, the fact of feeling an item being pressed against his body may be uncomfortable to the patient.
Further, and very importantly, the size of the examination space is already very small, providing just enough room for receiving a patient, and this in itself may already pose problems in the case of obese patients. A movable carrier plus carrier mount, arranged in the examination space, further reduces the “free” room in the examination space.
Further, since the local coil in this system is positioned by pushing it against the patient's body, the actual position of the local coil is not defined in advance since it is determined by the size of the patient.
Further, especially in the case of whole-body imaging, it is generally desirable to have receive coils positioned at the side of the patient. In the embodiment shown in said publication, only one movable carrier is located in the upper area of the bore.